Bulletin
149—Field-trip guide to the geochronology of El Malpais National
Monument and the ZuniBandera volcanic field, New Mexico

This field guide grew out of the 1993 Quaternary Dating
Field Conference held in Grants, New Mexico, in April 1993. The conference,
sponsored by the New Mexico Geochronology Research Laboratory (NMGRL) and
the U.S. National Park Service, was convened to consider using the Zuni-Bandera
volcanic field, including El Malpais National Monument and El Malpais National
Conservation Area, as a test area for evaluating, calibrating, and improving
the application of Quaternary dating techniques. Improvement of the accuracy
and precision of these dating techniques is one of the major goals of the
NMGRL, which was established in 1992 as a collaborative effort of the N
Bureau of Mines and Mineral Resources, the NMIMT, and the Los Alamos National
Laboratory. Much of the land that a visitor will cross in the two days of
this field trip is owned by the Ramah Navajo, Zuni, Laguna, and Acoma tribes,
or is under the jurisdiction of the U.S. National Park Service or the Bureau of
Land Management. Their permits are required for sample collection.

The Zuni-Bandera volcanic field is located
in west-central NM south of the town of Grants. It lies near the center
of the Jemez zone or lineament that extends at least from central Arizona
to northeastern NM. The Jemez lineament is a major flaw in the Earth's crust
along which volcanoes have erupted for the past 16 Ma. Approximately 100
volcanoes have been identified in the Zuni-Bandera volcanic field. The youngest
volcanic activity along the lineament occurred in the area now encompassed
by El Malpais National Monument, where volcanoes have erupted from about
7003 ka. Because of the youthfulness of these lava flows, they are
well preserved, providing the Monument visitor with spectacular views of
volcanic landforms and the geologist with an ideal area for investigating
volcanic processes.

One of the more important aspects of research
in the ZuniBandera volcanic field is the geochronology of the many
volcanic eruptions. Determination of the absolute ages of these basaltic
volcanoes is fundamental to other geological, geochemical, and isotopic
studies. Although there are several dating techniques that should be applicable
to volcanic rocks of this age, their results have rarely been compared in
a single area, and, where this has been attempted, they are often not in
agreement. The Quaternary Dating Field Conference was convened to address
this problem by assembling a group of researchers interested in applying
their specific techniques to a common sequence of lava flows with the goal
of improving their accuracy and precision, with the hope of improving our
understanding of the significance of the results obtained by the different
techniques. A secondary objective was the refinement of the geochronology
of the area.